A study was made of whether the administration of magnesium or ethmozin to conscious dogs with normal plasma Mg levels (0.75 +/- 0.06 mmol/l) terminates ouabain-induced ventricular tachycardia, an arrhythmia which is dependent on triggered activity resulting from delayed afterdepolarizations. Sustained monomorphic ventricular tachycardia was induced by programmed ventricular pacing during the continuous iv. infusion of ouabain to animals with a formaldehyde-induced permanent complete AV block. A single dose of MgSO4 (100 mg/kg b.w. iv.) abolished only those ventricular tachycardias with cycle lengths > 325 ms (inverse use-dependency); ventricular tachycardias with cycle lengths < 320 ms were merely slowed, despite the fact that the increase in plasma Mg levels was considerable and comparable in both groups (3.9 +/- 1.6 and 4.8 +/- 1.9 mmol/l). In contrast, ethmozin (2 mg/kg b.w. iv.) terminated both fast and slow ventricular tachycardias. It is concluded that ethmozin (moricizine) is more effective than the traditionally used MgSO4 in terminating ventricular tachycardia resulting from cardiac glycoside toxicity.